1. Field of the Invention
The present invention relates to a surface grinding method and a mirror polishing method for a thin plate such as a semiconductor silicon wafer (hereinafter also simply referred to as wafer) by an infeed type surface grinder.
2. Description of Related Art
In semiconductor wafer processing, a method has heretofore been adopted in which after a sliced wafer is chamfered along its peripheral portion, the sliced wafer is further subjected to lapping and etching in the order and thereafter, a surface thereof is mirror-polished.
While, in the etching step, generally a total of about 40 xcexcn has been removed on both surfaces in order to eliminate a work damage caused by lapping, the etching is a cause to reduce a flatness of a wafer in the final stage, in which mirror polishing is applied, since a flatness of a wafer is degraded by the etching.
Hence, in recent years, surface grinding has been adopted instead of lapping or after the etching in order to correct a flatness. Since, in surface grinding, there arises no work damage as deep as in lapping, a surface-ground wafer can be polished directly without any etching or only after very light etching, which amounts to a removal, for example, of 4 to 5 xcexcm in total on both surface. Therefore, adoption of surface grinding has an advantage to improve a wafer flatness, as compared with a conventional way.
In a case where a thin disc such as a semiconductor wafer is surface-ground, an infeed type surface grinder 12 as shown in FIG. 1 has very recently been employed. The surface grinder 12, which will be detailed later, has a construction and operating relations between constituents such that two circular tables 14 an 16, opposite to each other and one on the other, which are driven and rotate independently from each other, are arranged so that the peripheral end portion 18 of an upper table 14 coincides with the axial center 20a of a rotary shaft 20 of a lower table 16 all time, the two circular tables 14 and 16 being located so as to be shifted sideways from each other; not only is a grinding stone 22 held fixedly on a lower surface of the upper table 14, but a wafer (W) is fixed on an upper surface of the lower table 16; the tables 14 and 16 arranged one on the other are rotated relatively to each other; and at least one table is pressed on the other while at least one table is moved in a vertical direction, so that a surface of the wafer (W) is ground.
In a case where an infeed type surface grinder 12 as described above is adopted, there arises generally some error in parallelism between a rotary shaft 24 of the upper table and the rotary shaft 20 of the lower table and for this reason, trails only in an upper half surface or a lower half surface of the grinding stone 22 are observed on a ground surface of the wafer (W) at a constant pitch (e) in the form of grinding striations 26 comprising recesses and protrusions. The pitch (e) of the grinding striations 26 changes according to grinding conditions so as to be large (FIG. 2A) or small (FIG. 2B).
There has been a problem in connection with the surface grinding, since the grinding striations 26 cannot be removed in a mirror polishing, following the surface grinding, in which a regular stock removal of 10 xcexcm is effected and it is necessary to polish off a surface portion of the wafer by 20 to 30 xcexcm on one surface in order to fully eliminate the striations 26.
It has been experienced that deep pits occur locally on surfaces of the wafer (W) in lapping and the pits cannot be removed even in etching, which requires polishing-off of the order of 10 xcexcm. Since polishing-off of 10 xcexcm or deeper not only reduces productivity of a polishing step but deteriorate a flatness, compared with a conventional process, such increase in removal of polishing-off has to be avoided.
The present inventors have conducted serious studies from various angles on a surface grinding method by which grinding striations remaining on surfaces of a wafer caused in surface grinding using an infeed type surface grinder are produced so as to be able to be removed by polishing-off of 10 xcexcm or less and as a result, have acquired findings that there is a correlation between a pitch of grinding striations and a polishing-off depth to remove the striations and, in the course of further studies, that a polishing-off depth can be restricted to 10 xcexcm or less regardless of a diameter of a wafer if a pitch of grinding striations is adjusted to a given value or less. The present invention has been made based on such findings.
It is an object of the present invention to provide a surface grinding method by which grinding striations are produced so that the striations can fully be removed by a polish-off amount less than required in a conventional way in mirror polishing following surface grinding using an infeed type surface grinder.
In order to solve the above described problem, a surface grinding method of the present invention is directed to a surface grinding method for a wafer in which two circular tables, opposite to each other, which are driven and rotate independently from each other, are arranged so that the peripheral end portion of one table coincides with the axial center of a rotary shaft of the other table all time, the two circular tables being located so as to be shifted sideways from each other; not only is a grinding stone held fixedly on an opposite surface of the one table, but the wafer is fixed on an opposite surface of the other table; the two tables are rotated relatively to each other; and at least one table is pressed on the other while at least one table is relatively moved in a direction, so that a surface of the wafer is ground, wherein the surface of the wafer is ground while controlling a pitch of grinding striations produced across all the surface of the wafer processed by the grinding stone to be 1.6 mm or less.
A resinoid grinding stone that has some elasticity is preferred as a grinding stone held fixedly on the opposite surface of the one table. A number of the grinding stone is preferred to be of a fine grain size of #2000 or higher.
In order to control a pitch of the grinding striations to be 1.6 mm or less, the following two ways can be selected; a rotation number of a wafer in spark-out is adjusted or a rotation number of a wafer and a returning speed in escape are adjusted.
An additional way for the control of a pitch of the grinding striations is possibly adopted in which a rotation number (rotation rate) of the wafer during at least one rotation of the wafer just before a grinding stone in escape moves away from the wafer is adjusted.
A mirror polishing method for a wafer of the present invention is characterized by that a wafer that has been surface-ground by the above described surface grinding method receives mirror polishing. With this mirror polishing method for a wafer, there can be obtained a mirror polished wafer from which grinding striations are fully removed by a polishing-off amount less than in a conventional way.
The reason why a difference in polishing-off amount arises according to a pitch of grinding striations is considered to be that when a pitch of grinding striations is large, a polishing pad 30 is put into contact with a wafer surface so that the pad 30 covers closely along a surface contour of recesses and protrusions constituting grinding striations, as shown in FIG. 3A and thereby, the recesses and protrusions are hard to be erased, whereas when the pitch is small, the polishing pad is put into closer contact with the protrusions than with the recesses, as shown in FIG. 3B, which enables the surface contour to be flattened with ease. Based on such an estimated mechanism for flattening, a polishing-off amount can be reduced regardless of a diameter of a wafer by controlling a pitch of grinding striations to be equal to or smaller than a specific value.
A value of a pitch of grinding striations can be expressed by a formula: 2xcfx80r/[(a rotation number of a grinding stone)/(a rotation number of a wafer)], wherein r indicates a wafer radius. Therefore, to control a pitch of grinding striations to be 1.6 mm or less can be realized by controlling a rotation number of a grinding stone or a rotation number of a wafer.
Since a grinding stone, however, rotates at a high speed, to control the rotation number is likely very difficult from a mechanical viewpoint and therefore, it is preferred to control the pitch by a rotation number of a wafer.
On the other hand, if a returning speed in escape is small (for example, 0.01 xcexcm/sec or less) when an elastic grinding stone is used, an effect similar to that in spark-out can be obtained since the grinding stone is kept in contact with a wafer for a time.
Spark-out means a state in which a grinding stone and a wafer are both rotating after grinding-off of a given amount is completed and a feed of a grinding stone is ceased and escape means to move a grinding stone in a direction in which the grinding stone moves away from the wafer, the grinding stone and the wafer previously being in a state of spark-out.
As described above, according to the present invention, in surface grinding using a surface grinder, when a pitch of grinding striations in the peripheral portion of a wafer is adjusted to be a given value or less, grinding striations on a wafer surface can fully be eliminated by a polish-off amount less than in a conventional way, which can achieve a great effect enabling increase in productivity and improvement of a wafer flatness to be realized.